First stellar abundances in NGC 6822 from vlt-uves and keck-hires spectroscopy

We have obtained the first high-resolution spectra of individual stars in the dwarf irregular galaxy NGC 6822. The spectra of the two A-type supergiants were obtained at the Very Large Telescope and Keck Observatories, using the Ultraviolet-Visual Echelle Spectrograph and the High Resolution Echelle Spectrometer, respectively. A detailed model atmospheres analysis has been used to determine their atmospheric parameters and elemental abundances. The mean iron abundance from these two stars is 〈[Fe/H]〉 = -0.49 ± 0.22 (±0.21),⁶ with Cr yielding a similar underabundance, 〈[Cr/H]〉 = -0.50 ± 0.20 (±0.16). This confirms that NGC 6822 has a metallicity that is slightly higher than that of the SMC and is the first determination of the present-day iron group abundances in NGC 6822. The mean stellar oxygen abundance, 12 + log (O/H) = 8.36 ± 0.19 (± 0.21), is in good agreement with the nebular oxygen results. Oxygen has the same underabundance as iron, 〈[O/ Fe]〉 = +0.02 ± 0.20 (±0.21). This O/Fe ratio is very similar to that seen in the Magellanic Clouds, which supports the picture that chemical evolution occurs more slowly in these lower mass galaxies, although the O/Fe ratio is also consistent with that observed in comparatively metal-poor stars in the Galactic disk. Combining all of the available abundance observations for NGC 6822 shows that there is no trend in abundance with galactocentric distance. However, a subset of the highest quality data is consistent with a radial abundance gradient. More high-quality stellar and nebular observations are needed to confirm this intriguing possibility.